1. Field of the Invention
The present invention relates to a flash photographing system, and more particularly to an automatic light output controlable flash device.
2. Description of the Prior Art
An electronic flash device in which light exposure to an object is initiated in synchronism with a full-open state of an exposure aperture, an illumination light reflected by the object and transmitted through an imaging lens of a camera is measured by an automatic light output control circuit in the camera and light emission of a flash bulb is stopped when an integrated value of a photo-current of a photo-sensor of the automatic light output control circuit reaches a predetermined level, has been known.
There are two methods for the automatic light output control circuit to initiate the integration of the light reflected by the object. In a first method, a voltage change in the circuit when the flash bulb starts to emit the light is detected to synchronize the start of the integration of the light output control circuit to the voltage change. In a second method, the light output control circuit is activated by a mechanical switch in synchronism with the full-open state of the aperture of a shutter of the camera.
However, there is no interchangeability between a first type of camera which adopts the first method and a second type of camera which adopts the second method. In the second method which uses the mechanical switch, it is difficult for the automatic light output control circuit to operate in synchronism with the full-open state of the aperture and the automatic light output control circuit starts the integration before the flash device actually emits the light. As a result, a proper exposure is not attained by an effect of an external light in a daylight synchronization photographing.